Mixing Ratio of Air Calculator
Free calculate humidity ratio (mixing ratio) from vapor pressure and total pressure. Get instant, accurate results with our easy-to-use calculator.
Input Parameters
Standard atmospheric: 1013.25 hPa
Results
Enter parameters to calculate
What is Mixing Ratio?
The mixing ratio (also called humidity ratio or specific humidity) is the mass of water vapor per unit mass of dry air. It's expressed in grams of water vapor per kilogram of dry air (g/kg) or as a dimensionless ratio (kg/kg).
Mixing ratio is an absolute measure of moisture content - it doesn't depend on temperature like relative humidity does. It's particularly useful in meteorology and HVAC applications because it directly relates to the amount of water vapor in the air.
The mixing ratio is calculated from the vapor pressure of water and the total atmospheric pressure. It's a fundamental parameter in psychrometrics, used to understand air properties and design air conditioning systems.
Mixing Ratio Formula
Where:
- • r = Mixing ratio (kg/kg or g/kg)
- • e = Vapor pressure of water vapor (hPa, Pa, or mmHg)
- • P = Total atmospheric pressure (same units as e)
- • 0.622 = Ratio of molecular weights (M_water / M_air ≈ 18.015 / 28.965)
From dew point: Calculate saturation vapor pressure at dew point temperature, then use formula above.
From relative humidity: e = RH × e_sat(T), where e_sat is saturation vapor pressure at air temperature.
How to Calculate
-
1
Determine vapor pressure
If given directly, use it. If given dew point, calculate saturation vapor pressure at dew point. If given relative humidity, calculate e = RH × e_sat(T).
-
2
Convert units to consistent
Ensure vapor pressure and total pressure are in the same units (hPa, Pa, or mmHg).
-
3
Calculate mixing ratio
r = 0.622 × e / (P - e). Multiply vapor pressure by 0.622, divide by (total pressure - vapor pressure).
-
4
Convert to desired units
Result is in kg/kg. Multiply by 1000 to get g/kg (grams per kilogram of dry air).
Practical Examples
Example 1: From Vapor Pressure
e = 17.04 hPa, P = 1013.25 hPa.
Solution:
r = 0.622 × 17.04 / (1013.25 - 17.04)
r = 0.622 × 17.04 / 996.21
r ≈ 0.0106 kg/kg = 10.6 g/kg
Example 2: From Dew Point
Dew point: 15°C, P = 1013.25 hPa. Saturation vapor pressure at 15°C ≈ 17.04 hPa.
Solution:
e = 17.04 hPa (saturation at dew point)
r = 0.622 × 17.04 / (1013.25 - 17.04)
r ≈ 10.6 g/kg
Applications
Meteorology
Understanding atmospheric moisture, calculating humidity, and analyzing weather patterns. Mixing ratio is a key parameter in weather forecasting.
HVAC Systems
Designing air conditioning systems, calculating dehumidification requirements, and understanding psychrometric processes.
Industrial
Controlling humidity in manufacturing processes, ensuring product quality, and maintaining optimal environmental conditions.
Education
Teaching psychrometrics, understanding air properties, and demonstrating humidity calculations in atmospheric science.
Frequently Asked Questions
What's the difference between mixing ratio and relative humidity?
Mixing ratio is absolute (mass of water per mass of dry air) and doesn't change with temperature. Relative humidity is the ratio of actual to saturation vapor pressure and changes with temperature. Same mixing ratio can give different RH at different temperatures.
Why is the constant 0.622?
0.622 is the ratio of molecular weights: M_water (18.015 g/mol) / M_dry_air (28.965 g/mol) ≈ 0.622. This comes from the ideal gas law applied to water vapor and dry air components.
What is saturation mixing ratio?
Saturation mixing ratio is the maximum mixing ratio at a given temperature and pressure. It's calculated using saturation vapor pressure: r_sat = 0.622 × e_sat(T) / (P - e_sat(T)).
How does altitude affect mixing ratio?
Lower pressure at altitude means (P - e) is smaller, so mixing ratio increases for the same vapor pressure. However, air at altitude is typically drier, so actual mixing ratios are often lower than at sea level.
Can mixing ratio exceed saturation?
No! Mixing ratio cannot exceed saturation mixing ratio. If it did, condensation would occur until the air reaches saturation. Supersaturation can occur briefly in clouds, but it's unstable.